how can I pause Spotlight indexing?

Well, if your files are changing then Spotlight has to make changes. Without it you cannot find files on your computer. You can stop indexing completely by opening Spotlight preferences, clicking on the Privacy tab, then drag you disk icon into the window list. Removing it will turn Spotlight back on. You will not be able to use Spotlight to find files, folders, etc. while it is in the Privacy list. If you use Time Machine you will not be able to use Spotlight to find and restore files.

Since indexing is done in the background and stops when you start using the computer, there is very, if any, little slowdown caused by it.

It's constantly indexing my files.

Then the index or the things it is indexing are corrupt.

First, boot into Recovery and Repair the disk (not Repair Disk Permissions). It is on the First Aid tab.

If that doesn't calm down the indexing, erase the entire index.

The simple way is to go to Spotlight System Preferences and add you hard drive to the privacy tab. Wait about 10 seconds, then remove it from the list.

It will reindex the entire hard drive. The time it takes to finish should be under an hour.

If it is still indexing all the time, brute force erase the index with this command in Terminal (just copy and paste--otherwise you'll likely screw it up, then return):

sudo mdutil -E /

It will post a warning about using sudo, then ask for your password. When you type, you won't see anything. Just type the whole password and hit return.

Spotlight should then start indexing from scratch.